Kristen Brown -  Exponential Distance Dependence of Photoinitiated Stepwise Electron Transfer in Donor–Bridge–Acceptor Molecules: Implications for Wirelike Behavior

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      Ricks, A. B.; Brown, K. E.; Wenninger, M.; Karlen, S. D.; Berlin,   Y. A.; Co, D. T.; Wasielewski, M. R., Exponential Distance   Dependence of Photoinitiated Stepwise Electron Transfer in   Donor–Bridge–Acceptor Molecules: Implications for Wirelike   Behavior J. Am. Chem. Soc. 2012, 134 (10), 4581-4588.


        Donor–bridge–acceptor (D–B–A) systems in which a   3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) chromophore and   a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked   by oligomeric 2,7-fluorenone (FNn)   bridges (n    = 1–3) have been synthesized. Selective photoexcitation of DMJ-An   quantitatively produces DMJ+•-An–•,   and An–•    acts as a high-potential electron donor. Femtosecond transient   absorption spectroscopy in the visible and mid-IR regions showed   that electron transfer occurs quantitatively in the sequence:   DMJ+•-An–•–FNn–NI   → DMJ+•-An–FNn–•–NI   → DMJ+•-An–FNn–NI–•.   The charge-shift reaction from An–•to   NI–•    exhibits an exponential distance dependence in the nonpolar   solvent toluene with an attenuation factor (β) of 0.34   Å–1,   which would normally be attributed to electron tunneling by the   superexchange mechanism. However, the FNn–•    radical anion was directly observed spectroscopically as an   intermediate in the charge-separation mechanism, thereby   demonstrating conclusively that the overall charge separation   involves the incoherent hopping (stepwise) mechanism. Kinetic   modeling of the data showed that the observed exponential   distance dependence is largely due to electron injection onto the   first FN unit followed by charge hopping between the FN units of   the bridge biased by the distance-dependent electrostatic   attraction of the two charges in D+•–B–•–A.   This work shows that wirelike behavior does not necessarily   result from building a stepwise, energetically downhill redox   gradient into a D–B–A molecule

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